A DISCUSSION OF THE GEOLOGICAL AGES OF THE DABIE,HONGAN AND YINGSHAN GROUPS

The Dabie, Hongan and Yingshan groups discussed in this paper refer to the pre-Sinian strata distributed extensively at the southern feet of the Tongbai and Dabie mountains. On the basis of the data obtained in regional geological surveys in recent years, the Group of Metamorphic Strata has preliminarily made a comprehensive study of the differences in rock association, primary rock formation, metamorphism, tectonism and magmatism of the Dabie, Hongan and Yingshan groups. According to the results of the study combined with the data on isotopic ages and microfossils, it is considered that the three groups represent respectively three different stages of geological development and reflect the polycyclic characteristics of the crustal evolution during the pre-Sinian period in the area. The Dabie Group often constitutes the core of the ancient structure and represents a metamorphic rock series that has undergone strong regional metamorphism and migmatization. It consists mainly of gneisses, plagioclase amphibolites and various types of migmatites. The protoliths might possibly be a volcanic-sedimentary sequence composed of basic-intermediate acid volcanic rocks and small amounts of argillo-arenaceous, high-alumina, siliceous-ferruginous and calc-magnesian carbonate rocks. According to the character of the metamorphic mineral association, the rocks are considered to be of widespread, homogenously recrystallized, medium-pressure almandine amphibolite facies. Granites fromed by granitization evidently exhibit features of anatexis. Zircon from the Dabie Group yields a U-Pb apparent age of 1952-2424 m. y., and the age of the upper intersection point used on Wetherill’s concordia is 2900 m. y.; thus the Dabie Groub is inferred to be of Archeanearly Proterozoic age. Therefore it follows that the Dabie movement roughly corresponds to the Wutai Movement. The Hongan Group is a metamorphic rock series characterized by medium-pressure greenschist facies and composed of low-grade metamorphic schists. It lies unconformably on the Dabie Group. It is inferred that its protoliths might be a eugeosynclinal volcanic-sedimentary sequence composed of spilite-quartz keratophyre and terrigenous clastic-carbonate sediments. There was no phenomenon of regional migmatization in the main, and intermediate-acid magmatism was by far weaker than that of the Dabie Group. The Hongan Group often forms tight folds of northwest trend. The attitude of strata of the Hongan Group is obviously inconsistent with that of the Dabie Group in the area from Jiming of Yingshan county to Chenhangou of Susong county and in Zhupozhai of Qichun county, and Huangmailing of Dawu county. In some places fossil crust of weathering and basal conglomerates have been observed on unconformable surfaces, and reddingite beds of industrial value occur commonly. The U-Pb zircon apparent ages of the Hongan Group used are 1762 m. y., 1486 m. y. and 1850 m. y., respectively. According to the isotopic age data combined with microfossil data, the age of the Hongan Group might be inferred to be Middle Proterozoic, corresponding to the Changchengian-Jixianian periods. The contact of the Yingshan Group with the underlying Dagushi Group （corresponding to the Jixian system） is angular unconformities, （its contact with the Hongan Group is faulted（?））; while its contact with the overlying pre-Sinian Doushantuo Formation is parallel unconformities. It is represented by a volcanic-sedimentary-metamorphic series of the spilite-keretophyre formation type, belonging to the low-temperature portion of medium-high pressure greenschist facies. Its U-Pb zircon apparent ages used average 969 m. y. and the Pb-Pb isochron age is 852 m. y., so it is infereed that the age of the Yingshan Group might corresponds to the Qingbaikouan period of the late Middle Proterozoic.